Process fluid or gas bearings are now being utilized in an increasing number of diverse applications. These fluid bearings generally comprise two relatively movable elements with a predetermined spacing therebetween filled with a fluid such as air, which, under dynamic conditions forms a supporting wedge sufficient to prevent contact between the two relatively movable elements.
More recently, improved fluid bearings, particularly gas bearings of the hydrodynamic type, have been developed by providing foils in the space between the relatively movable bearing elements. Such foils, which are generally thin sheets of a compliant material, are deflected by the hydrodynamic film forces between adjacent bearing surfaces and the foils thus enhance the hydrodynamic characteristics of the fluid bearings and also provide improved operation under extreme load conditions when normal bearing failure might otherwise occur. Additionally, these foils provide the added advantage of accommodating eccentricity of the relatively movable elements and further provide a cushioning and dampening effect.
The ready availability of relatively clean process fluid or ambient atmosphere as the bearing fluid makes these hydrodynamic, fluid film lubricated, bearings particularly attractive for high speed rotating machinery. While in many cases the hydrodynamic or self-acting fluid bearings provide sufficient load bearing capacity solely from the pressure generated in the fluid film by the relative motion of the two converging surfaces, it is sometimes necessary to externally pressurize the fluid between the bearing surfaces to increase the load carrying capacity. While these externally pressurized or hydrostatic fluid bearings do increase the load carrying capacity, they do introduce the requirement for an external source of clean fluid under pressure.
Illustrative of hydrodynamic and/or hydrostatic bearing patents assigned to the same Assignee of this application are U.S. Pat. Nos.: 3,215,479; 3,215,480; 3,366,427; 3,375,046; 3,382,014; 3,434,762; 3,467,451; 3,511,544; 3,560,064; 3,615,121; 3,635,534; 3,642,331; 3,677,612 and 3,893,733.
In the operation of these fluid film foil gas bearings, at startup and rundown and in some cases even at higher speeds, there is actual rubbing contact between the foils and the bearing surfaces with respect to which there is relative movement. This may be between the foils and a shaft or bushing or in the case of thrust bearings, with respect to a thrust plate or runner. In any case there may also be rubbing contact where individual foils or foil stiffener elements overlap.
In order to lower the startup friction and prevent wear or galling at these contact surfaces, wherever they may be, the foils, usually a thin compliant metallic material, are often uniformly coated with a dry lubricating material which is generally softer than the contacting surface. The lubricant material may be of one kind or a mixture of such substances as fluorinated hydrocarbon polymer, graphite, or molybdenum disulfide, all of which are characteristically difficult to make adhere to any metal substrate. Usually they are mixed with a binder to produce better adhesion and other substances to increase their hardness, temperature, and wear resistance. In addition, the foil surface may be etched by various methods such as acid dipping or grit blasting or the coating may be applied by plasma spray or ion deposition means. Sometimes a primer coating with lesser lubricating qualities is applied first. Examples of patents specifically directed to foil coatings are U.S. Pat. Nos. 3,677,612 and 4,005,914 and British Pat. No. 821,954.
Such composites and their wear products tend to produce higher friction co-efficients and may produce a type of debris that doesn't easily clear itself from the bearing. The added materials may also detract from the ability of pure fluorinated hydrocarbons to resist chemical attack.
Even though the lubricating layer needed at the contact surface is quite thin, these uniform thickness coatings must start initially with an appreciable thickness to store enough lubricant to compensate for the depletion of the lubricant layer from repeated starts as the coating will wear in a way to conform to any surface curvature. As transparent spots develop on the metal surface, the surface may still be thinly but invisibly lubricated by wiping as long as fresh material keeps abrading and transferring across the bearing surface. Finally the bare spots will increase in area until there is no coating left to spread and the lubricating coating is thus depleted to the point of failure. There are many limitations however to simply increasing the thickness of the coatings, including the bearing contours and the flexing of the compliant foils.
Over the life of a foil bearing, the wear of the lubricant coating will increase the bearing clearance or "sway space" between the movable or rotatable elements. In the case of a non-linear spring loaded foil bearing journal, the preload and spring stiffness will decrease and thus allow the shaft runout to increase. To compensate for this anticipated wear, the initial preload and stiffness must generally be greater. This in turn adds to the startup friction and minimum shaft speed required for the foil bearing to become airborne.
Since the thickness of a single coating that is applied by spraying or dipping is limited to obtain the necessary smooth surface free of runs, it typically may take 2 to 3 coats to obtain 0.001 in. of thickness in some well known composite coatings based on fluorinated hydrocarbon polymers (such as Teflon-S). Curing the coating also takes longer and greater care when the coating is thicker.
Another limitation to increasing the coating thickness is its effect on the deformation "imprint" which impedes starting like a wedge under a wheel. This causes higher brakeaway friction and initial wear especially in a foil journal bearing. As the coating wears thinner it tends to act "harder" and wear more slowly. Harder coatings facilitate the transition from sliding to rolling motion needed to get started.